1. Field of the Invention
The invention relates to a III-V group compound semiconductor light-emitting diode and, more particularly, a GaP green light emitting diode.
2. Description of the Prior Art
GaP crystals or GaAsP (gallium arsenide phosphide) crystals of the III-V group compound semiconductors have been used as material for light emitting diodes which have also been widely used for various types of display devices. The light-emitting diode using the GaP crystal emits light with wavelengths ranging from 550 nm to 720 nm, i.e. green to red colors. The light emitting diode using GaAsP crystal emits light with wavelengths from 580 to 670 nm, i.e. yellow to red colors. In the green light emitting diode or the yellow light emitting diode, nitrogen doped atoms are used for the luminescence center impurity. Green light emitting GaP diodes in the light emission elements with nitrogen doping are prepared as follows. An n-type GaP layer is formed on an n-type GaP substrate by the liquid phase epitaxial growth method or the vapor epitaxial growth method. Then, a p-type GaP layer is further formed on the n-type GaP layer by the diffusion method or the liquid-phase epitaxial growth method. Nitrogen atoms serving as the luminescence center are added to the n-type GaP layer and the p-type GaP layer. For example, in a paper "IEEE Transactions on Electron Devices" Vol. ED-24 No. 7, on pages 951 to 955, published in July 1977, nitrogen atoms are added by using ammonia NH.sub.3 or gallium nitride (GaN). The right column on page 954 in this article describes that there is a correlation between the nitrogen concentration (N.sub.T) and the donor concentration (N.sub.D) toward the growth direction in the n-type layer. From this description, it is estimated that, in order to improve the light emission efficiency (N.sub..eta.), the donor concentration (N.sub.D) should be decreased to increase nitrogen concentration in the vicinity of the p-n junction. As apparently seen from the diagram of FIG. 6 on page 953 in the article, when the N.sub.T is 1.times.10.sup. 18 /cm.sup.3 or more, the life time .tau..sub.G of minority carriers is shortened to be below 150 nsec and thus the light emission efficiency .eta..sub.G is reduced. Incidentally, in order to improve the light emission efficiency, it is necessary to improve the lifetime of minority carriers in the light emission region (the portion of the n-type GaP layer in the vicinity of the p-n junction) and to increase the luminescence center (nitrogen atoms) concentration in the light emission region. This is treated in "Aprll. Phys. Letters, Vol. 22 No. 5, on pp 227 to 229 (particularly equation (1) on page 229). This paper also describes that the lifetime of minority carriers is at maximum, 100 nsec when the donor concentration in the n-type GaP layer is approximately 1.times.10.sup.17 /cm.sup.3, and the nitrogen concentration N.sub.T at this time is 1.times.10.sup.19 /cm.sup.3. (However, a paper "J. Electron Mat." Vol. 1, pp 39 to 53, published in 1972 describes that the correct value of N.sub.T is approximately 1/4 of the above N.sub.T value). This paper, however, does not refer to a means to improve the lifetime of minority carriers and to improve the N.sub.T in the portion of the n-type GaP layer in the vicinity of the p-n junction, although FIG. 2 on page 228 denotes that the lifetime of the minority carries in the n-type GaP may be improved by decreasing the N.sub.D . In this case, however, it is saturated when the N.sub.D is below 1.times.10.sup.17 /cm.sup.-3.
In a paper "J. Electrochem. Soc." Vol. 122, No. 3, pages 407 to 412, published in 1975, it is described that, when the temperature in the epitaxial growth of the n-type GaP layer is low and the temperature for p-n junction formation is decreased to 850.degree. C., the lifetime of the minority carrier is improved to be 200 nsec. In this case, the N.sub.T is 2.times.10.sup.18 /cm.sup.3, N.sub.D 7.6.times.10.sup.16 /cm.sup.3 (or, 6.times.10.sup.16 /cm.sup.3 or 1.7.times.10.sup.16 /cm.sup.3). Accordingly, various conditions mentioned above may be satisfied to some extent. The light emission efficiency of the diodes prepared by this method is 0.35% at maximum under the forward current of 25 A/cm.sup.2 and with the epoxy moulding of the device (FIG. 2).
In a paper "J, Electrochem. Soc." Vol. 119, No. 12, pages 1780 to 1782, particularly FIG. 2 on page 1782, published in Dec., 1972, an experiment and calculation are performed to find what number of nitrogen atoms enter the GaP growth layer with respect to growth temperature, when the liquid phase growth method is employed as in the previous paper. FIG. 2 of this paper shows that, when the growth temperature of GaP is at 960.degree. C., the N.sub.T is approximately 2.times.10.sup.18 /cm.sup.3 at maximum and it becomes larger as the temperature becomes higher.
From the facts mentioned above, it is inferred that, in order to improve the light emission efficiency, it is preferable that the N.sub.D of the n-type GaP layer is set low, for example, 6.times.10.sup.16 /cm.sup.3, the N.sub.T approximately 2.times.10.sup.18 /cm.sup.3 and the lifetime of the minority carrier is set at 200 nsec. On the inference, when considering a description that the light emission efficiency is 0.33% (the device is moulded) at a current of 7 A/cm.sup.2 in a paper "Journal of Crystal Growth 27", pp 183 to 192, particularly on page 191, the device described is estimated that N.sub.D of the n-type GaP layer is low, N.sub.T is large and the lifetime of minority carriers is long. As seen from FIG. 6 on page 189 in the paper, when an n-type GaP layer is formed on the n-type GaP substrate by the liquid phase epitaxial growth method, H.sub.2 S and ammonia NH.sub.3 are added in gas phase into a Ga solution. In mid course, growth and the addition of H.sub.2 S are stopped, and it is subjected to purging at about 970.degree. C. to remove sulfur S from the Ga solution. Zinc (Zn) and ammonia (NH.sub.3) are added into the solution of Ga, and then growth of the p-type GaP layer is started in liquid phase epitaxial process. As a consequence, it is considered that the lifetime of the minority carriers is long and the light emission efficiency improved.
FIG. 7 on page 189 of the same article illustrates only a baking condition in which donor impurity concentrations change in accordance with the change of the thickness of the growth layer, the N.sub.D of the top portion thereof decreasing by the disappearance of sulfur (S) in the purging at 970.degree. C.
The GaP green light emitting element thus manufactured exhibits the maximum efficiency of light emission under the manufacturing condition described in the above-mentioned document. That is, the average light emission efficiency is approximately 0.15% (with being molded) under 25 A/cm.sup.2.